The invention relates to a catheter of the species named in the preamble of claim 1 to measure chemical parameters and in particular to be inserted into biological tissue, fluids or the like.
Catheters of this kind are widely known. They are called microdialysis catheters and are used to measure certain chemical parameters inside the biological tissue, usually the human body, using microdialysis. A typical example of such chemical parameters is the content in blood sugar in the subcutaneous tissue. The principle of this measurement is illustratively described in the German patent document C2 40 01 760 and in the patent document WO 95/32746 and consists in inserting a double-lumen probe, fitted at its tip with a semi-permeable portion, through the skin. A transfering solution is pumped through a duct in the catheter into this probe, flowing past the semi-permeable membrane and in the process absorbs blood sugar from the sub-cutaneous tissue. This transfering solution enriched with blood-sugar is then removed from the body through a second catheter duct and when outside said body will be mixed with a glucose/oxidase solution and fed to a sensor measuring the reaction products of the glucose oxidation reaction. Continuous measurement of blood sugar at relatively short delays is made possible by such systems.
Microdialysis probes with which to carry out the above measurements are known for instance in the patent documents U.S. Pat. No. 5,106,365 and WO 95/20983. A catheter defined in the preamble of claim 1 is known from the German patent document A1 33 42 170. It is known therein as a dialysis probe and comprises a dialysis membrane and ducts for the perfusion through the membrane. The dialysis membrane is enclosed by a housing supporting the membrane while partly being clear from it and also is more rigid than the membrane itself. The ducts are diametrically opposite within the catheter and terminate inside the tubular membrane. The relatively rigid housing serves to prevent the tubular membrane from being crushed after the catheter has been inserted into the biological tissue and thus to preserve the membrane""s operation and the flow through it. Because of feature of the supporting housing, this known catheter is complex and hence expensive in its manufacture and moreover entails a comparatively large diameter. It incurs a further drawback in that the transfer flow inside the tubular membrane is non-uniform over its surface.
Aside the drawback that the partly permeable membrane is unevenly wetted by the flow of liquid and the entailed substantial restriction of the exchange surface, the probe of this species incurs the further drawback it cannot be operated under suction because in that case a reduced pressure would arise inside the tubular membrane and would cause it to collapse. When operating under pressure, a dead volume is incurred, causing the test results to be delayed. Moreover the total volume within the tubular membrane of this known probe is comparatively large and accordingly the ratio of exchange surface to volume is low and hence the concentration in drained liquid is low and measurement must be carried out at a lower level.
The objective of the invention is to create a catheter of the above kind which shall be free of the drawbacks of the state of the art, that is, which shall be manufacturable in especially simple and economical manner, of which the diameter and the dead volume shall be small, and which shall allow measurement without undue time delays and at higher measurement levels.
The task of the invention is solved by the disclosure of the features of claim 1.
The basic concept of the invention is to make the feed and return ducts in the catheter coaxial and to use the semi-permeable tubular membrane as the outer wall of the external duct at the end of the catheter. This design step already offers a low dead volume within the tubular membrane because most of the inner volume is filled by the cross-section of the inner wall of the external duct which at the same time is the external wall of the inner duct. In order to secure uniform cross-section of the external duct in spite of its small, clear width and simultaneously to prevent collapse of the tubular membrane in the region of the catheter end, another feature of the invention provides spacers mutually bracing the walls of the radially external feed duct or of the radially external return duct. A radial aperture is present at the outer catheter end to connect the inner and external ducts and as a result the external duct is reliably passing transfering liquid over the full length of the tubular membrane and effective drainage of the ingredients transmitted by the membrane from the surrounding tissue shall be assured.
The spacers may assume various designs. In one embodiment, the spacers are ribs formed on the inner wall of the external feed or return duct. The ribs maybe manufactured in very simple manner when extruding the inner catheter part. The part forming the external of the outer duct then may be pulled or pushed in the form of a tube over the inner part. However the outer and inner parts and the ribs may be extruded integrally.
In the simplest case the ribs run in the longitudinal direction of the catheter. However in especially appropriate manner the ribs shall be helical. In this manner intensive wetting of the inside surface of the semi-permeable membrane and accordingly evacuation of the ingredients transmitted by the semi-permeable membrane is assured. For a given volume of transfering liquid, the helical design of the ribs furthermore increases the flow of the transfering liquid and further the entrainment of the ingredients transmitted by the semipermeable membrane is thus made possible. Appropriately the ribs fill the full radial space of the external duct, as a result of which leakage flows between the outer edges of the ribs and the adjacent wall will be precluded.
In principle the diameter of the tubular membrane is arbitrary. In particular it may be larger than the catheter per se, for instance when the tubular membrane is slipped by its end away from that of the catheter on the catheter shank and is affixed at that place. In practice, however, the tubular membrane and the catheter appropriately shall be of approximately the same diameters.
In one suitable embodiment of the invention, the external tubular part is made of membrane material over its full length and is fitted with an impermeable layer as far as the catheter""s front end zone. This design allows eliminating bonding the tubular part 12 to the membrane 16 in the region of the bonding surface 14 and hence eliminating a correspondingly complex manufacturing stage. Instead the external tubular part is made of membrane material over its full length and the single remaining manufacturing stage will be to affix the sealing layer on the inner or outer wall of the external tubular part, for instance by a simple dipping procedure.